The invention relates to a measuring instrument for time-variable magnetic fields or field gradients, to electrical resistance elements, which can be used as core components in the measuring instrument, and to a measuring system comprising a measuring instrument or an electrical resistance element according to the invention.
Superconducting quantum interference devices (SQUIDs) are at present the most sensitive sensors for magnetic fields. The dynamic range thereof, however, is limited. If the measuring site is permeated not only by the time-variable magnetic field to be measured, but also by an interference field, which is static and greater by several orders of magnitude, or which can be varied only slowly, the sensor is often saturated by the interference field alone. The measurement signal itself is only a very small modulation in a high background signal caused by the interference field.
Flux transformers are employed so as to separate the measurement signal from the background signal. Using a pick-up coil, these convert the time-variable component of the magnetic flux generated by the field to be measured in this pick-up coil into electric current. This current feeds a magnetic field source, which typically is a coil (input coupling coil), which thereupon generates an auxiliary magnetic field. This auxiliary field is measured by the sensor itself, which is typically a SQUID.
To this end, superconducting flux transformers are typically employed for the most sensitive measurements at low frequencies so as to minimize signal losses (J. E. Zimmermann, N. V. Frederick, “Miniature Ultrasensitive Superconducting Magnetic Gradiometer and Its Use in Cardiography and Other Applications”, Appl. Phys. Lett. 19, 16 (1971)). The disadvantage is that successive interference components, which are due to highly static as well as very slowly variable (time constant>10 min) magnetic interference fields, accumulate in the circuit composed of the pick-up coil and magnetic field source. This progressively worsens the dynamic range and sensitivity of the measuring system.
Normally conducting flux transformers are known, for example, from (T. Q. Yang, Kenichiro Yao, Daisuke Yamaski, Keiji Enpuku, “Magnetometer utilizing SQUID picovoltmeter and cooled normal pickup coil”, Physica C 426-431, 1596-1600 (2005)) and (D. F. He, H. Itozaki, M. Tachiki, “Improving the sensitivity of a high-Tc SQUID at MHz frequency using a normal metal transformer”, Superconductor Science and Technology 19, pp. 231-234 (2006)). The disadvantage is that, at approximately 10 μs, the relaxation time constants of these flux transformers are too short, so that high losses occur at low frequencies below approximately 100 kHz and the achievable measurement results are no longer meaningful. At the same time, they introduce tremendous noise into the measurement signal in this frequency range.
A flux transformer is known from (H. Dyvorne, J. Scola, C. Fermon, J. F. Jacquinot, M. Pannetier-Lecoeur, “Flux transformers made of commercial high critical temperature superconducting wires”, Review of Scientific Instruments 79, 025107 (2008)), the pick-up coil and input coupling coil of which are each made of tape-shaped wires and which are connected to each other by a tape-shaped double-circuit line. The normally conducting soldering points between the coils and the double-circuit line dissipate the interference components accumulating in superconducting coils. The disadvantage is that the sensitivity of this array is insufficient, notably for biomagnetic and geomagnetic measurements.